A major problem associated with electrical distribution cable is its tendency, over a period of time, to fail due to the progressive degradation of its insulation. The degradative processes involved in the failure of cables are correlated with two "treeing" phenomena. "Electrical treeing" is the product of numerous electrical discharges in the presence of strong electrical fields which eventually lead to the formation of voids within the insulation material. These voids resemble the trunk and branches of a tree in profile under microscopic observation, from which the descriptive terminology derives. As the trees formed by this process grow, they provide further routes along which corona discharges can occur, the cumulative effect being electrical breakdown of the insulation. Electrical treeing generally occurs when large voltages are imposed on the cable. The degradative results of the electrical treeing process can be precipitous such that the electrical cable can break down in a relatively short period of time.
The second type of treeing, known as "water treeing," is observed when the insulation material is simultaneously exposed to moisture and an electric field. This mechanism is much more gradual than electrical treeing, requiring an extended period of time to cause the degree of damage that affects the insulation characteristics of the distribution cable. However, since water treeing occurs at considerably lower electrical fields than required for the formation of electrical trees, this phenomenon is a leading cause of reduced service life of cables which allow water entry to the insulator region, whether through diffusion or some other mechanism.
Efforts have been made to prepare cables which are resistant to water treeing by incorporating an anti-tree additive (e.g., certain organosilanes) directly into the insulation composition of the cable at time of manufacture. Additionally, it is known to minimize deterioration by limiting the amount of water which can enter the cable interior by introducing a curable, e.g., silicone "water block" composition into the cable's interior and subsequently crosslinking this composition therein. However, these methods only address the issue of how to inhibit the formation of trees and the associated deterioration of insulation integrity. Thus, the skilled artisan will recognize that a vast network of underground cable is already in place wherein the cable either has not been treated according to one of these procedures or it has degenerated significantly despite such efforts and is, therefore, subject to premature failure.
As a partial answer to industry's desire to extend the useful life of existing underground cables, it has been found that certain liquid tree retardants can be introduced into the cable's interior to partially restore the insulation performance. For example, phenylmethyldimethoxysilane can be pumped in to fill the intersticial void space between the stranded conductor and the conductor shield of the cable: the silane is then allowed to diffuse into the cable's insulation to fill the electrochemical trees therein. This restorative procedure can then be followed by the introduction of a water block composition, as mentioned above.
It will be appreciated, however, that these techniques require the pumping of liquids having viscosities in the range of about one to 100 cP at 25.degree. C. through the interior of the cable. The success of this procedure therefore relies on the availability of an unobstructed path in the cable under consideration. In a typical field situation, however, a partial or total blockage of the cable segment is often encountered; this vitiates the utility of the above described restorative methods. Such a blockage can arise in several ways. For example, corrosion of the conductor strands due to the ingress of water or the accumulation of standing water in the conductor area can lead to partial or total blockage of the interstitial void space. More typically, the obstruction is due to the presence of one or more splices, elbows or other connections along the cable segment. These too can partially or completely block the flow of liquid and thereby thwart any intended restorative attempts. Additionally, such connections can develop leaks over time or they may contain leaks from improper installation. Leaks are also undesirable since they can promote electrical breakdown, facilitate the penetration of water into the cable and result in the waste of restorative fluids when the latter are introduced. Leaks may also be formed when a cable faults electrically, in which case an electrical discharge burns a hole in the cable insulation causing a failure and leak therein. For the purposes herein, the generic term "disruption" will be used to refer to such leaks, partial obstructions or complete obstructions, individually or in combination.
In general, an electrical utility will have no knowledge of the presence or location of these disruptions in any given cable segment selected for restoration (e.g., it has been in service for considerable time or has actually failed). Since the systematic elimination of such disruptions prior to cable restoration generally necessitates the unearthing of buried cable, their locations must be ascertained as accurately as possible in order to avoid the excessive time and expense of unproductive excavation. Moreover, since much of this excavation takes place on the lawns of private residences, there is also strong motivation to minimize guesswork and the associated complaints of irate home owners. Toward this end, the art has made only limited progress. For example, it is known to determine whether a cable segment is completely blocked by applying a gas flow to one end thereof in series with a rotometer by noting the eventual cessation of flow.
Two relatively sophisticated electronic methods which can precisely locate a splice, elbow or termination along the cable segment are also currently available. The first, time domain reflectometry (TDR), is based on a radar-like principle wherein an electrical pulse is propagated along the conductor of the cable and the impedance mismatch of, e.g., a splice within the cable segment generates an electronic reflection or echo which can be plotted on an oscilloscope and used to calculate its location. A second method uses a radio tone generator to send a radio signal down the conductor of the cable segment, this signal being returned to the source via the concentric neutral wire of the cable. Perturbations in the strength of this signal due to the different conductor and neutral wire geometries associated with, e.g., a splice can then be analyzed to determine the locations thereof. Even though the above mentioned electronic methods are quite accurate in locating a splice they are not useful in the determination of other types of disruptions. Nor can they establish the extent of blockage of splices elbows or terminations. Furthermore, when complete blockage of a given cable segment containing two or more such splices is established, these methods can not be used to determine which of these is the restrictive device. These electronic methods also can not locate a splice or elbow which is near an end of a cable segment.